Methods and apparatus for cleaning semiconductor wafers

ABSTRACT

A method for cleaning semiconductor substrate using ultra/mega sonic device comprising holding a semiconductor substrate by using a chuck, positioning a ultra/mega sonic device adjacent to the semiconductor substrate, injecting chemical liquid on the semiconductor substrate and gap between the semiconductor substrate and the ultra/mega sonic device, changing gap between the semiconductor substrate and the ultra/mega sonic device for each rotation of the chuck during the cleaning process. The gap can be increased or reduced by 0.5 /N for each rotation of the chuck, where λ is wavelength of ultra/mega sonic wave, N is an integer number between 2 and 1000. The gap is varied in the range of 0.5λn during the cleaning process, where λ is wavelength of ultra/mega sonic wave, and n is an integer number starting from 1.

FIELD OF THE INVENTION

The present invention generally relates to method and apparatus forcleaning semiconductor wafer. More particularly, relates to changing agap between an ultra/mega sonic device and a wafer for each rotation ofthe wafer during the cleaning process to achieve an uniform ultra/megasonic power density distribution on the wafer, which removes particlesefficiently without damaging the device structure on the wafer.

BACKGROUND

Semiconductor devices are manufactured or fabricated on semiconductorwafers using a number of different processing steps to create transistorand interconnection elements. To electrically connect transistorterminals associated with the semiconductor wafer, conductive (e.g.,metal) trenches, vias, and the like are formed in dielectric materialsas part of the semiconductor device. The trenches and vias coupleelectrical signals and power between transistors, internal circuit ofthe semiconductor devices, and circuits external to the semiconductordevice.

In forming the interconnection elements the semiconductor wafer mayundergo, for example, masking, etching, and deposition processes to formthe desired electronic circuitry of the semiconductor devices. Inparticular, multiple masking and plasma etching step can be performed toform a pattern of recessed areas in a dielectric layer on asemiconductor wafer that serve as trenches and vias for theinterconnections. In order to removal particles and contaminations intrench and via post etching or photo resist aching, a wet cleaning stepis necessary. Especially, when device manufacture node migrating to 65nm and beyond, the side wall loss in trench and via during is crucialfor maintaining the critical dimension. In order to reduce oreliminating the side wall loss, it is important to use moderate, dilutechemicals, or sometime de-ionized water only. However, the dilutechemical or de-ionized water usually is not efficient to remove particlein the trench and via. Therefore the mechanical force such as ultrasonic or mega sonic is needed in order to remove those particlesefficiently. Ultra sonic and mega sonic wave will apply mechanical forceto wafer structure, the power intensity and power distribution is keyparameters to control the mechanical force within the damage limit andat the same time efficiently to remove the particles.

Mega sonic energy coupled with nozzle to clean semiconductor wafer isdisclosed in U.S. Pat. No. 4,326,553. The fluid is pressurized and megasonic energy is applied to the fluid by a mega sonic transducer. Thenozzle is shaped to provide a ribbon-like jet of cleaning fluidvibrating at mega sonic frequencies for the impingement on the surface.

A source of energy vibrates an elongated probe which transmits theacoustic energy into the fluid is disclosed in U.S. Pat. No. 6,039,059.In one arrangement, fluid is sprayed onto both sides of a wafer while aprobe is positioned close to an upper side. In another arrangement, ashort probe is positioned with its end surface close to the surface, andthe probe is moved over the surface as wafer rotates.

A source of energy vibrates a rod which rotates around it axis parallelto wafer surface is disclosed in U.S. Pat. No. 6,843,257 B2. The rodsurface is etched to curve groves, such as spiral groove.

To uniformly apply right amount of mega sonic power to entire wafer iscritical for the cleaning process. If the mega sonic power is notuniformly applied on the wafer, the portion of wafer receiving less megasonic power will not be cleaned well, and leaving particles andcontamination on the portion of the wafer, and portion of waferreceiving extra mega sonic power may cause the damage of devicestructure on the wafer.

It is needed to have a better method for controlling the mega sonicpower density distribution on the wafer to clean particles andcontamination on surface of wafer or substrate with higher efficiencyand lower structure damages.

SUMMARY

One method of the present invention is to put a mega sonic deviceadjacent to front side of a rotating wafer during the cleaning process,and to increase the gap between the mega sonic device and the wafer foreach rotation of the wafer. The increment of the gap for each rotationof the wafer is a friction of half wavelength of mega sonic wave, andthe total increment of the gap is in the range of 0.5λN, where λ is thewavelength of mega sonic wave, and N is an integer number starting from1.

Another method of the present invention is to put a mega sonic deviceadjacent to front side of a rotating wafer during the cleaning process,and to reduce the gap between the mega sonic device and the wafer foreach rotation of the wafer. The reduction of the gap for each rotationof the wafer is a friction of half wavelength of mega sonic wave, andthe total reduction of the gap is in the range of 0.5 λN, where λ is thewavelength of mega sonic wave, and N is an integer number starting from1.

Another method of the present invention is to put a mega sonic deviceadjacent to back side of a rotating wafer, and to increase the gapbetween the mega sonic device and the wafer for each rotation of thewafer during the cleaning process. The increment of the gap for eachrotation of the wafer is a friction of half wavelength of mega sonicwave, and the total increment of the gap is in the range of 0.5 λN,where

is the wavelength of mega sonic wave, and N is an integer numberstarting from 1.

Another method of the present invention is to put a mega sonic deviceadjacent to back side of a rotating wafer, and to reduce the gap betweenthe mega sonic device and the wafer for each rotation of the waferduring the cleaning process. The reduction of the gap for each rotationof the wafer is a friction of half wavelength of mega sonic wave, andthe total reduction of the gap is in the range of 0.5 λN, where

is the wavelength of mega sonic wave, and N is an integer numberstarting from 1.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1D depict an exemplary wafer cleaning apparatus;

FIG. 2 depicts an exemplary wafer cleaning process;

FIGS. 3A-3B depicts another exemplary wafer cleaning process;

FIG. 4 depicts another exemplary wafer cleaning apparatus;

FIG. 5 depicts a cleaning method;

FIG. 6 depicts another exemplary wafer cleaning apparatus;

FIG. 7 depicts another exemplary wafer cleaning apparatus;

FIG. 8 depicts another exemplary wafer cleaning apparatus;

FIG. 9 depicts another exemplary wafer cleaning apparatus;

FIGS. 10A-10G depicts variety of shape of ultra/mega sonic transducers.

DETAILED DESCRIPTION

FIGS. 1A to 1B show the conventional wafer cleaning apparatus using amega sonic device. The wafer cleaning apparatus consists of wafer 1010,wafer chuck 1014 being rotated by rotation driving mechanism 1016,nozzle 1012 delivering cleaning chemicals or de-ionized water 1032, andmega sonic device 1003. The mega sonic device 1003 further consists ofpiezoelectric transducer 1004 acoustically coupled to resonator 1008.Transducer 1004 is electrically excited such that it vibrates and theresonator 1008 transmits high frequency sound energy into liquid. Theagitation of the cleaning liquid produced by the mega sonic energyloosens particles on wafer 1010. Contaminants are thus vibrated awayfrom the surfaces of the wafer 1010, and removed from the surfacesthrough the flowing liquid 1032 supplied by nozzle 1012.

As shown in FIG. 1C, in order to achieve the least reflection energy,the phase reflection wave r1 (from top of water film) must be oppositeto reflection R2 (bottom of water film), therefore water film thicknessmust equal to:d=nλ/2, n=1,2,3, . . .  (1)

Where, d is the thickness of water film or gap between mega-sonic device1003 and wafer 1010, n is an integer number, and λ is wavelength of megasonic wave in water. For example, for mega sonic frequency of 937.5 KHz, λ=1.6 mm, the d=0.8 mm, 1.6 mm, 2.4 mm, and so on.

FIG. 1D shows the relationship between gap d and mega sonic powerdensity measured by sensor 1002 as shown in FIG. 1A. Power densityvaries from valley value 20 w/cm2 to peak value 80 w/cm2 as gap sizeincrease 0.4 mm, and reach a full cycle in the gap increment of 0.8 mm(0.5λ). It is critical to maintain a gap precisely in order to keep auniform mega sonic power distribution on the entire wafer.

However, it is very difficult to keep a uniform gap in such precision inreality, especially when the wafer is rotation mode. As shown in FIG. 2,if wafer chuck 1014 is set not 100% vertical to surface of mega sonicdevice 2003, the gap between mega sonic device and surface of wafer 2010is reducing from edge of the wafer to center of the wafer. It will causenon uniform mega sonic power density distribution from edge of the waferto center of the wafer according to data shown in FIG. 1D.

Another possible gap variation is caused by rotation axis of chuck beingnot vertical to the surface of wafer 3010 as shown in FIGS. 3A and 3B.The wafer is wobbling when rotating, and FIG. 3B shows gap status afterrotating 180 degree from status as shown in FIG. 3A. The gap at edge ofwafer reduces from a biggest value as shown in FIG. 3A to smallest valueas shown in FIG. 3B. It will cause non uniform mega sonic power densitydistribution on wafer as wafer passing mega sonic device. All such nonuniform power distribution will either cause damage to device structureon the wafer and non uniform cleaning of the wafer.

In order to overcome non uniform power distribution caused by variationof gap during chuck rotation, the present invention discloses a methodas shown in FIG. 4. The gap between mega sonic device 4003 and wafer4010 is increased or reduced by lead screw 4005 and motor 4006 as chuck4014 rotating during cleaning process. Control unit 4088 is used tocontrol the speed of motor 4006 based on speed of motor 4016. For eachrotation of wafer 4010 or chuck 4014, control unit 4088 instructs motor4006 to move mega sonic device 4003 up or down:

z=0.5λ/N  (2)

Where, λ is wavelength of ultra/mega sonic wave, and N is an integernumber between 2 to 1000.

As shown further in detail in FIG. 5, when the gap increases for eachrotation of wafer or chuck, the mega sonic power density at the sameportion of wafer changes from P1 to P2. When the gap increases totalhalf wavelength of mega sonic wave, the power density varies a fullcycle from P1 to P11. The cycle starting point depends on the gapbetween mega sonic device and portion of wafer, however each portion onwafer will receive full cycle of power density when gap increases halfwavelength of mega sonic wave. In other words, even gap between megasonic device and wafer is not set uniformly due to reason described inFIG. 2, FIGS. 3A and 3B, each portion of the wafer will receive fullcycle of mega sonic power when mega sonic device moves up halfwavelength of mega sonic wave (about 0.8 mm for frequency of 937.5 kHz).This will guarantee each location of wafer to receive the same mount ofmega sonic power density including the same average power density, thesame maximum power density, and the same minimum power density. Theoperation sequence can be set as follows:

Process Sequence 1 (mega sonic frequency: f=937.5 kHz, and wavelength indeionized water=λ=1.6 mm):

Step 1: rotating wafer at speed of ω, and ω is in the range of 10 rpm to1500 rpm.

Step 2: move mega sonic device to adjacent to wafer with gap d, and d isin the range of 0.5 to 15 mm.

Step 3: turn on nozzle with deionized (DI) water or chemicals, and turnthe mega sonic device on.

Step 4: for each rotation of chuck, move mega sonic device up 0.5λ/N(mm), where N is an integer number and in the range of 2 to 1000.

Step 5: continue step 4 until mega sonic device moves up total 0.5 nλ(mm), where n is an integer number starting from 1.

Step 6: for each rotation of chuck, move mega sonic device down 0.5λ/N(mm), where N is an integer number and in the range of 2 to 1000.

Step 7: continue step 6 until mega sonic device moves down total 0.5nλ(mm), where n is an integer number starting from 1.

Step 8: repeat step 4 to step 7 until wafer is cleaned.

Step 9: turn off mega sonic devices, stop the DI water or chemicals, andthen dry the wafer.

Process Sequence 2 (mega sonic frequency: f=937.5 kHz, and wavelength indeionized water=λ=1.6 mm):

Step 1: rotating wafer at speed of co, and co is in the range of 10 rpmto 1500 rpm.

Step 2: move mega sonic device to adjacent to wafer with gap d, and d isin the range of 0.5 to 15 mm.

Step 3: turn on nozzle with deionized (DI) water or chemicals, and turnthe mega sonic device on.

Step 4: for each rotation of chuck, move mega sonic device up 0.5λ/N(mm), where N is an integer number and in the range of 2 to 1000.

Step 5: continue step 4 until mega sonic device moves up total 0.5λ(mm), where n is an integer number starting from 1.

Step 6: turn off mega sonic devices, stop the DI water or chemicals, andthen dry the wafer.

The frequency of transducer can be set at ultra sonic range and megasonic range, depending on the particle to be cleaned. The larger theparticle size is, the lower frequency should be used. Ultra sonic rangeis between 20 kHz to 200 kHz, and mega sonic range is between 200 kHz to10 MHz. Also frequency of mechanical wave can be alternated either oneat a time in succession or concurrently in order to clean different sizeof particles on the same substrate or wafer. If a dual frequency ofwaves are used, the higher frequency f₁ should be multiple integernumber of lower frequency f₂, and the transducer moving range should bethe 0.5λ₂n, increment or reduction of gap for each rotation of chuckshould be 0.5λ₁/N, which λ₂ is wavelength of the wave with the lowerfrequency f₂, λ₁ is wavelength of the wave with the higher frequency f₁,and N is an integer number between 2 to 1000, and n is an integer numberstarting from 1.

One example of chemicals being used to remove the particle andcontamination are shown as follows:

Organic Material Removal: H₂SO₄:H₂O₂=4:1

Particle Reduction: NH₄OH:H₂O₂:H₂O=1:1:5

Metal Contamination Removal: HCl:H₂O₂:H₂O=1:1:6

Oxide Removal: Oxide Removal=HF:H₂O=1:100

FIG. 6 shows another embodiment of wafer cleaning apparatus using a megasonic device according to the present invention. The embodiment issimilar to that shown in FIG. 4, except that chuck 6014 is movedvertically by lead screw 6005 and motor 6006. Control unit 6088 changesthe gap d between mega sonic device 6003 and wafer 6010 by moving chuck6014 up and down through lead screw 6005 and motor 6006.

FIG. 7 shows another embodiment of wafer cleaning apparatus using a megasonic device according to the present invention. The embodiment issimilar to that shown in FIG. 4, except that mega sonic device 7003 isplaced adjacently to the back side of wafer 7010, and is movedvertically by lead screw 7005 and motor 7006. Control unit 7088 changesthe gap d between mega sonic device 7003 and back side of wafer 7010 bymoving mega sonic device 7003 up and down through lead screw 7005 andmotor 7006. Mega sonic wave is transmitted to front side of wafer 7010and water film 7032 through water film 7034 and wafer 7010. Nozzle 7011supplies DI water or chemicals to maintain water film 7034 between megasonic device 7003 and back side of wafer 7010. The advantage of thisembodiment is to reduce or eliminate possible damage caused by megasonic wave to device structure on front side of wafer 7010.

FIG. 8 shows another embodiment of wafer cleaning apparatus using a megasonic device according to the present invention. The embodiment issimilar to that shown in FIG. 4, except that wafer 8010 is placed facedown, and a nozzle array 8018 is placed underneath of front side ofwafer 8010. Mega sonic wave is transmitted to front side of wafer 8010through water film 8032 and wafer 8010 itself. A nozzle array 8018sprays liquid chemicals or DI water on to front side of wafer 8010.

FIG. 9 shows another embodiment of wafer cleaning apparatus using a megasonic device according to the present invention. The embodiment issimilar to that shown in FIG. 4, except that surface of piezoelectrictransducer 9004 has an angle α to surface of wafer 9010. Resonator 9008is attached with piezoelectric transducer 9004, and mega sonic wave istransmitted to wafer through the resonator 9008 and DI water or chemicalfilm 9032. Process sequence 1, 2, and 3 can be applied here.

FIGS. 10A to 10G show top view of mega sonic devices according to thepresent invention. Mega sonic device shown in FIG. 4 can be replaced bydifferent shape of mega sonic devices 10003, i.e. triangle or pie shapeas shown in FIG. 10A, rectangle as shown in FIG. 10B, octagon as shownin FIG. 10C, elliptical as shown in FIG. 10D, half circle as shown inFIG. 10E, quarter circle as shown in FIG. 10F, and circle as shown inFIG. 10G.

Although the present invention has been described with respect tocertain embodiments, examples, and applications, it will be apparent tothose skilled in the art that various modifications and changes may bemade without departing from the invention.

What is claimed is:
 1. A method for cleaning a semiconductor substrateusing an ultra/mega sonic device, comprising: holding a semiconductorsubstrate by using a chuck, the chuck connected to a motor; positioningthe ultra/mega sonic device adjacent to the semiconductor substrate;executing a cleaning process using a control unit, the cleaning processincluding injecting a chemical liquid onto the semiconductor substrateand into a gap between the semiconductor substrate and the ultra/megasonic device using at least one nozzle, applying an ultra/mega sonicwave to the cleaning liquid using the ultra/mega sonic device, andchanging the gap between the semiconductor substrate and the ultra/megasonic device, and wherein the control unit controls the speed of thechuck and changes the gap between the semiconductor substrate and theultra/mega sonic device based on a value of a wavelength of theultra/mega sonic wave in the cleaning liquid and rotation of the chuck,and wherein during the cleaning process the control unit is configuredto adjust the gap between the semiconductor substrate and the ultra/megasonic device for each rotation of the chuck by 0.5λ/N until a totalchange of the gap is 0.5nλ to provide a uniform power densityapplication of the ultra/mega sonic device to the entire semiconductorsubstrate, wherein λ is the wavelength of the ultra/mega sonic wave, Nis an integer number between 2 to 1000, and n is an integer numberstarting from
 1. 2. The method of claim 1, wherein the gap is changed bymoving the ultra/mega sonic device in a direction vertical to thesemiconductor substrate.
 3. The method of claim 1, wherein the gap ischanged by moving the chuck in a direction vertical to the ultra/megasonic device.
 4. The method of claim 1, wherein the ultra/mega sonicdevice is positioned adjacent to a front side of the semiconductorsubstrate.
 5. The method of claim 1, wherein the ultra/mega sonic deviceis positioned adjacent to a back side of the semiconductor substrate. 6.The method of claim 5, wherein the chemical liquid is injected to afront side of the semiconductor substrate by a first nozzle of the atleast one nozzle placed adjacent to the front side of the semiconductorwafer, and the chemical liquid is injected to a back side of thesemiconductor substrate by a second nozzle of the at least one nozzleplaced adjacent to the back side of the semiconductor substrate.
 7. Themethod of claim 1, wherein sonic frequencies of the ultra/mega sonicdevice are dual frequencies.
 8. The method of claim 7, wherein the dualfrequencies comprises a high frequency f1 and a low frequency f2, andf1=Mf2, where M is an integer number starting from
 2. 9. The method ofclaim 6, wherein the chemical liquid is injected simultaneously on thefront side and back side of the semiconductor wafer by the first nozzleand the second nozzle, respectively.
 10. A method for cleaning asemiconductor substrate using an ultra/mega sonic device, comprising:holding a semiconductor substrate by using a chuck, the chuck connectedto a motor; positioning the ultra/mega sonic device adjacent to thesemiconductor substrate; executing a cleaning process using a controlunit, the cleaning process including injecting a chemical liquid ontothe semiconductor substrate and into a gap between the semiconductorsubstrate and the ultra/mega sonic device using at least one nozzle,applying an ultra/mega sonic wave to the cleaning liquid using theultra/mega sonic device, and changing the gap between the semiconductorsubstrate and the ultra/mega sonic device, and wherein the control unitcontrols the speed of the chuck and changes the gap between thesemiconductor substrate and the ultra/mega sonic device based on a valueof a wavelength of the ultra/mega sonic wave in the cleaning liquid androtation of the chuck, and wherein during the cleaning process thecontrol unit is configured to adjust the gap between the semiconductorsubstrate and the ultra/mega sonic device for each rotation of the chuckby 0.5λ/N to provide a uniform power density application of theultra/mega sonic device to the entire semiconductor substrate, wherein λis the wavelength of the ultra/mega sonic wave, and N is an integernumber between 2 to
 1000. 11. The method of claim 10, wherein the gap ischanged by moving the ultra/mega sonic device in a direction vertical tothe semiconductor substrate.
 12. The method of claim 10, wherein the gapis changed by moving the chuck in a direction vertical to the ultra/megasonic device.
 13. The method of claim 10 wherein the ultra/mega sonicdevice is positioned adjacent to a front side of the semiconductorsubstrate.
 14. The method of claim 10, wherein the ultra/mega sonicdevice is positioned adjacent to a back side of the semiconductorsubstrate.
 15. The method of claim 14, wherein the chemical liquid isinjected to a front side of the semiconductor substrate by a firstnozzle of the at least one nozzle placed adjacent to the front side ofthe semiconductor wafer, and the chemical liquid is injected to a backside of the semiconductor substrate by a second nozzle of the at leastone nozzle placed adjacent to the back side of the semiconductorsubstrate.
 16. The method of claim 10, wherein the gap is varied in therange of 0.5λn during the cleaning process, where n is an integer numberstarting from
 1. 17. The method of claim 10, wherein sonic frequenciesof the ultra/mega sonic device are dual frequencies.